This invention relates in general to cold forging a workpiece, and in particular, to a method of cold forging a ball joint housing having a non-circular opening.
Forging is a manufacturing process by which metal is plastically deformed beyond its yield point under great pressure into high-strength parts. The process is normally (but not always) performed hot by preheating the metal to a desired temperature. It is important to note that the forging process is entirely different from the casting (or foundry) process, in which the metal used is melted, then poured or injected into a die.
There are four basic methods used to make a forged part: (1) Impression Die Forging, (2) Open Die Forging, (3) Seamless Rolled Ring Forging, and (4) Cold Forging. Impression die forging plastically deforms metal between two dies that contain a precut profile of the desired part. Parts weighing from ounces to 60,000 lbs. can be made using this process. Commonly referred to as closed-die forging, impression-die forging of steel, aluminum, titanium and other alloys can produce an almost limitless variety of 3-D shapes that range in weight from mere ounces up to more than 25 tons. Impression-die forging is routinely practiced using hydraulic presses, mechanical presses and hammers, with capacities up to 50,000 tons, 20,000 tons and 50,000 lbs. respectively. As the name implies, two or more dies containing impressions of the part shape are brought together as forging stock undergoes plastic deformation. Because metal flow is restricted by the die contours, this process can yield more complex shapes and closer tolerances than open-die forging processes. Additional flexibility in forming both symmetrical and non-symmetrical shapes comes from various preforming operations (sometimes bending) prior to forging in finisher dies. The geometry for a part can range from some of the easiest to forge simple spherical shapes, block-like rectangular solids, and disc-like configurations to the most intricate components with thin and long sections that incorporate thin webs and relatively high vertical projections like ribs and bosses. Although many parts are generally symmetrical, others incorporate all sorts of design elements (flanges, protrusions, holes, cavities, pockets, etc.) that combine to make the forging very non-symmetrical. In addition, parts can be bent or curved in one or several planes, whether they are basically longitudinal, equi-dimensional or flat. Most engineering metals and alloys can be forged via conventional impression-die processes, among them: carbon and alloy steels, tool steels, and stainless steels, aluminum and copper alloys, and certain titanium alloys. Strain-rate and temperature-sensitive materials (magnesium, highly alloyed nickel-based superalloys, refractory alloys and some titanium alloys) may require more sophisticated forging processes and/or special equipment for forging in impression dies. Larger parts up to 200,000 lbs. and 80 feet in length can be hammered or pressed into shapes this way.
Open-die forging can produce forging from a few pounds up to more than 150 tons. Called open-die because the metal is not confined laterally by impression dies during forging, this process progressively works the starting stock into the desired shape, most commonly between flat-faced dies. In practice, open-die forging comprises many process variations, permitting an extremely broad range of shapes and sizes to be produced. In fact, when design criteria dictate optimum structural integrity for a huge metal component, the sheer size capability of open-die forging makes it the clear process choice over non-forging alternatives. At the high end of the size range, open-die forging is limited only by the size of the starting stock, namely, the largest ingot that can be cast. Practically all forgeable ferrous and non-ferrous alloys can be open-die forged, including some exotic materials like age-hardening superalloys and corrosion-resistant refractory alloys. Open-die shape capability is indeed wide in latitude. Not unlike successive forging operations in a sequence of dies, multiple open-die forging operations can be combined to produce the required shape. At the same time, these forging methods can be tailored to attain the proper amount of total deformation and optimum grain-flow structure, thereby maximizing property enhancement and ultimate performance for a particular application. Forging an integral gear blank and hub, for example, may entail multiple drawing or solid forging operations, then upsetting. Similarly, blanks for rings may be prepared by upsetting an ingot, then piercing the center, prior to forging the ring.
Seamless rolled ring forging is typically performed by punching a hole in a round piece of metal (creating a donut shape) and then rolling and squeezing (or in some cases, pounding) the donut into a thin ring. Ring diameters can range from a few inches to 30 feet. Rings forged by the seamless ring rolling process can weigh less than 1 lb. up to 350,000 lbs. Performance-wise, there is no equal for forged, circular-cross-section rings used in energy generation, mining, aerospace, off-highway equipment and other critical applications. Seamless ring configurations can be flat (like a washer), or feature higher vertical walls (approximating a hollow cylindrical section). Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall-thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing. In fact, seamless tubes up to 48-in. diameter and over 20-ft long are extruded on 20 to 30,000-ton forging presses. Even though basic shapes with rectangular cross-sections are the norm, rings featuring complex, functional cross-sections can be forged to meet virtually any design requirements. Aptly named, these contoured rolled rings can be produced in thousands of different shapes with contours on the inside and/or outside diameters. A key advantage to contoured rings is a significant reduction in machining operations. Not surprisingly, custom-contoured rings can result in cost-saving part consolidations. Compared to flat-faced seamless rolled rings, maximum dimensions (face heights and O.D.'s) of contoured rolled rings are somewhat lower, but are still very impressive in size. High tangential strength and ductility make forged rings well-suited for torque- and pressure-resistant components, such as gears, engine bearings for aircraft, wheel bearings, couplings, rotor spacers, sealed discs and cases, flanges, pressure vessels and valve bodies. Materials include not only carbon and alloy steels, but also non-ferrous alloys of aluminum, copper and titanium, as well as nickel-base alloys.
Most forging is done as hot work, at temperatures up to 2300 degrees F, however, a variation of impression die forging is cold forging. Cold forging encompasses many processes--bending, cold drawing, cold heading, coining, extrusions and more, to yield a diverse range of part shapes. The temperature of metals being cold forged may range from room temperature to several hundred degrees. Cold forging encompasses many processes such as bending, cold drawing, cold heading, coining, extrusion, punching, thread rolling and more to yield a diverse range of part shapes. These include various shaft-like components, cup-shaped geometry's, hollow parts with stems and shafts, all kinds of upset (headed) and bent configurations, as well as combinations of these components. Most recently, parts with radial flow like round configurations with center flanges, rectangular parts, and non-axisymmetric parts with 3- and 6-fold symmetry have been produced by warm extrusion. With cold forging of steel rod, wire, or bar, shaft-like parts with 3-plane bends and headed design features are not uncommon. Typical parts are most cost-effective in the range of 10 lbs. or less: symmetrical parts up to 7 lbs. readily lend themselves to automated processing. Material options range form lower-alloy and carbon steels to 300 and 400 series stainless steel, selected aluminum alloys, brass and bronze. There are times when warm forging practices are selected over cold forging especially for higher carbon grades of steel or where in-process anneals can be eliminated. Often chosen for integral design features such as built-in flanges and bosses, cold forging is frequently used in automotive steering and suspension parts, antilock-braking systems, hardware, defense components, and other applications where high strength, close tolerances and volume production make them an economical choice.
In the cold forging process, a chemically lubricated bar slug is forced into a closed die under extreme pressure. The unheated metal thus flows into the desired shape. There are three basic cold forging process operations: (1) Forward Extrusion, (2) Backward Extrusion, and (3) Upsetting, or Heading. In forward extrusion, the metal flows in the direction of the ram force to reduce slug diameter and increases its length to produce parts such as stepped shafts and cylinders. In backward extrusion, the metal flows back and around the descending punch opposite to the ram force to form hollow and cup-shaped parts. In upsetting, the metal flows at right angles to the ram force, increasing diameter and reducing length to gather the metal in the head and other sections along the length of the part to form flattened parts, such as fasteners and the like.
The cold forging process has several distinct advantages over other types of metal fabrication techniques. One advantage is the capability to form net shape parts and reduce material usage and scrap up to 50% over other forming processes. High throughput rates and the elimination of slow secondary machining operations contribute to reduced part cost over fabrication methods. Also, cold forging can flow metals into a wide variety of the most complicated shapes with great precision. Many parts produced by cold forging cannot be duplicated economically by any other type of metal fabrication. In addition, cold forging offers a unique ability to effect metal grain structure, size and orientation through work hardening. Thus, electrical/mechanical characteristics, hardness and other mechanical properties can be enhanced with the cold forging process. Further, cold forging can achieve better smoothness without secondary operations than other types of metal fabrication processes. This contributes to both the appearance and the economy of the part. Finally, cold forging can utilize a wide variety of metals to produce parts that perform as specified.
One of the variety of complicated shapes that can be formed using the cold forging operation is a net shape or near net shape housing for a ball joint having a circular opening in the bottom of the housing. The circular opening in the bottom of the housing permits movement of the toggle or shaft in the x- and y-directions to provide a 360 degree range of motion for the toggle or shaft.
Although cold forging can be used to form a variety of complicated shapes, cold forging a ball joint housing having a non-circular opening, rather than circular opening was believed to be impossible. One difficulty expected to be encountered is that the punch and stool would come in contact with each other and therefore be destroyed by the enormously large capacity of the press. Another expected difficulty is that the incorporation of the non-circular opening during cold forging would tend to misshape the final product, thereby making it slightly non-circular along the longitudinal axis of the non-circular opening. Consequently, in the conventional manufacture of ball joint housings, the housing is cold forged with a circular opening, rather than a non-circular opening, and then milled to form the non-circular opening, resulting in a more time-consuming and costly practice.